1. Field of the Invention
This invention relates to a combustion chamber structure for diesel engines in which a precombustion chamber body forming a precombustion chamber is provided in a cavity formed in a piston.
2. Description of the Prior Art
A swirl chamber type engine having swirl chambers has been developed for the purpose of improving the combustion of an engine. Such a twirl chamber type engine has swirl cambers formed in a cylinder head or piston heads, communication ports communicating with each other the swirl chambers and primary chambers formed in cylinders, and fuel injection nozzles for spraying a fuel in an atomized state into the swirl chambers, a gaseous mixture being formed by vortex currents flowing into the swirl chambers via the communication ports and the fuel injected into the swirl chambers, to carry out the primary combustion of the gaseous mixture, gases including flames and unburnt gaseous mixture being then ejected from the swirl chambers into the primary chambers via the communication ports to carry out the secondary combustion of the gaseous mixture. There is a precombustion chamber structure disclosed in Japanese Patent Laid-Open No. 112613/1990 as an example of such a swirl chamber type engine.
Japanese Utility Model Laid-Open No. 93141/1987 discloses a piston comprising a ceramic crown and an aluminum skirt. In this piston, an outer circumferential portion of the ceramic crown in which a cavity is formed is fitted in a head portion of the cylindrical aluminum skirt, and these two parts are fixed to each other by providing a cast iron ring and a plastic fluidized material in a clearance between the outer circumferential portion of the combustion chamber and the head portion.
Japanese Patent Laid-Open No. 26111/1993 discloses a heat insulating piston. In this piston, the heat insulating characteristics of a portion between a piston head and a piston skirt are secured, whereby the occurrence of cracks in and the scattering of the heat insulating member provided on a joint portion of the two parts in prevented.
The emission regulation has become severer in recent years, and various types of combustion chambers for diesel engines have been developed. In order to improve the fuel efficiency, a combustion chamber comprising an open chamber is used in many cases. An open chamber for a diesel engine is formed so as to scatter a fuel uniformly in a combustion chamber by using a high-pressure fuel injection pump, increase an explosive force by increasing a combusting rate, and improve the engine performance. However, in an open chamber, the density of air in the interior of a combustion chamber is large, and the temperature of the air around the atomized gaseous mixture becomes high with a rate of generation of NOx increasing to cause a big problem to arise.
In a precombustion chamber type engine having swirl chambers, two-stage combustion is carried out, in which a fuel is ignited and burnt in precombustion chambers, gases including flames and unburnt gaseous mixture being then ejected from the precombustion chambers into primary chambers via communication ports, the secondary combustion of the gaseous mixture being carried out in the primary chambers. Therefore, the generation of NOx decreases advantageously. However, in a precombustion chamber type engine, a combustion period becomes long, and the engine performance lowers. The conceivable reasons why the performance of a precombustion chamber type engine lowers are as follows. Namely, the reasons include (1) since precombustion chambers in a precombustion chamber type engine are positioned at circumferential portions of cylinders, the time of arrival of gases including flames and unburnt gaseous mixture, which are ejected from the precombustion chambers into primary chambers, at the cylinder wall surfaces becomes uneven, and the ejection period of time is long, so that a combustion period in the primary chambers cannot be reduced, (2) the speed of vortex current in the precombustion chambers in such a engine increases, and a heat loss from the precombustion chambers to the outside becomes large, and (3) since the cross-sectional area of the communication ports via which the primary and precombustion chambers in such an engine communicate with each other is as small as around 1.4%, a restriction loss of the communication ports becomes large, and output level does not become high.
The durability of a diesel engine is a very important property thereof for keeping the lifetime of the engine long. In the conventional techniques for making a combustion chamber of a diesel engine, combustion chamber manufacturing methods of casting cast iron into an aluminum alloy, forming alumina (Al.sub.2 O.sub.3) by subjecting the surface of an alumina alloy to an oxidation process, or mixing ceramic fiber in an aluminum alloy are used. The production of a diesel engine has a problem of forming a combustion chamber to a heat insulating structure having a high strength and a high durability and capable of being produced simply.